1. Field of the Invention
The present invention relates to a linear motor.
2. Description of the Related Art
Differing from rotary motors, in linear motors, the armature has a limited length in the thrust direction. As a result, it has been known that the thrust of the linear motor has a pulsating component, called cogging force, due to the structure at the opposite ends of the armature with respect to the thrust direction. The cogging force may cause reduction in the positioning precision of the linear motor. Accordingly, various measures have been proposed to reduce the cogging force, including modifying the shapes of the teeth at both ends or the like.
JP2003-299342A discloses a linear motor in which the tip of the tooth located at either end in the armature moving direction is partly cut off on the side on which no neighboring tooth is formed, so that the length of the tip of the tooth with respect to the armature moving direction is shorter than that of the other teeth.
JP2005-223997A discloses a linear motor in which the surfaces of the two pole teeth located at both ends of the core includes: a first slope inclined so that the gap between the pole tooth surface and the permanent magnet array becomes gradually greater as it is farther away from the neighboring tooth; and a second slope inclined so that the gap between the pole tooth surface and the permanent magnet array becomes gradually smaller as it is farther away from the first slope.
JP2005-102487A discloses a linear motor in which the magnetic pole surfaces of the pole teeth located at both ends of the core have an arc-curved surface such that its gap with the permanent magnet array becomes greater as it is farther away from the neighboring tooth.
JP2008-125322A discloses a linear motor including an armature which has a plurality of main teeth arranged parallel to each other with a pair of auxiliary teeth at both ends of the main teeth, in which the auxiliary teeth are arranged obliquely to the main teeth.
JP2004-364374A discloses a linear motor including an armature which has a plurality of main teeth arranged parallel to each other with a pair of auxiliary teeth at both ends of the main teeth, in which the shapes of the auxiliary teeth are changed in accordance with the length of the armature core in the aligning direction of field poles or the number of the main teeth.
JP2011-188709A discloses a linear motor including an armature which has a plurality of main teeth wound with windings as well as a pair of auxiliary teeth at opposite ends of the armature core, in which the auxiliary teeth are formed so that the dimensions perpendicular to the moving direction are changed along the direction of travel.
WO2009/044748A discloses a linear motor including a stator and a rotor, in which the tip of each tooth of the rotor has a portion projecting in the moving direction of the rotor and is divided into multiple areas in a direction perpendicular to the moving direction of the rotor and parallel to the magnetic pole surface of the permanent magnets of the stator, and the amount of projection in the moving direction is modified at least in one of the neighboring areas.
According to the above known art, there are many factors to be considered in determining the size of the teeth located at the ends of the armature core, so that it is difficult to obtain an optimal solution to reduce the cogging force. Further, the known art can provide only limited effects in reducing the cogging force. Therefore, there is a need for a linear motor that can effectively reduce the cogging force due to the structure at the ends of the armature.